The present invention relates in general to thermal control circuits, and, more precisely, to circuits for thermally controlling integrated power devices (smart-power devices) such as driving inductive loads.
Power stages driving inductive loads require, in certain applications, thermal control for gradually decreasing the current when the temperature increases (soft thermal shut down) and until a steady state condition is reached. At steady state, the power dissipated by the Joule effect equals the rate of heat dissipated in the surroundings.
For example, this kind of thermal control is typically required in integrated power devices employed in electronic ignition systems for engines of motor vehicles. In these applications a soft thermal shut down control circuit avoids the occurrence of an abrupt turning off of the power device in series with the coil upon reaching an abnormally high temperature because of possible anomalous functioning. Generation of sparks at the plugs and undesired detonations are thus prevented.
A typical prior art circuit that performs such a function as depicted in FIG. 1A includes functional elements that play a role during an ON phase of the power stage. Instead of a single bipolar transistor, several transistors in a Darlington configuration, or even in a three-stage configuration, if the current absorption by control circuits must be small, may be used.
When a turn on signal xe2x80x9cINxe2x80x9d switches from a low logic state to a high logic state, the current generator Iref is enabled. This generator, through a current amplifier A1I with a constant gain A1, turns on the power stage by forcing a drive current Ib. In these conditions, the collector current Ic of the power stage increases and its variation depends on the value of the inductance of the load and on the supply voltage Vbat.
A current limiter, CURRENT LIMITER, is normally present for limiting the current Ic at a maximum value Icl to ensure the functioning of the device within the allowed temperature range. The current limiter is activated when the voltage drop at the nodes of a sensing resistance RSENS overcomes a certain value and it drains a current Ilim equal to the difference between the current Iref and a replica, scaled by a factor A1, of the driving current Ib that is equal to the ratio between the limitation current Icl at the prescribed working conditions and the gain of the power device. In these conditions, the power stage will work in the direct biasing region of its current characteristic, thus causing strong power dissipation because of the Joule effect.
Diagrams of the main signals as a function of the time and of the temperature are also depicted in FIG. 1B, at the left and at the right of the dashed line portion thereof, respectively. Should the turn on signal remain high because of any malfunction, starting a current limitation phase, the power stage will dissipate a far greater power than under nominal working conditions, thus increasing the temperature of the integrated circuit. When a certain pre-established temperature TSOFT is reached, the soft thermal shut down circuit, SOFT THERMAL SHUT DOWN, activates itself and absorbs a current Ith linearly increasing with the temperature with a coefficient K1.
The current variations in each block must satisfy the 1st Kirchhoff""s law at node A, that is:             Ib              A        ⁢                  xe2x80x83                ⁢        1              =          Iref      -      Isum        ;      xe2x80x83    ⁢      Isum    =                  I        ⁢                  xe2x80x83                ⁢        lim            +      Ith        ;      xe2x80x83    ⁢      Iref    =    cost    ;
In the temperature range from TSOFT and TSTART, the current Ith increases of the same amount as Ilim decreases, thus keeping Isum constant, while the current amplifier xe2x80x9cA1Ixe2x80x9d provides the drive current Ib that is necessary to force the required current Ic in the power integrated transistor.
On the contrary, for a temperature T greater than TSTART an increase of the current Ith implies an increase of Isum with a consequent decrease of the input current of the current amplifier A1I. Therefore, a decrease of the current Ib and thus a decrease of the maximum current Icl that may flow in the power transistor is obtained. As it is possible to note, there is a temperature TSTOP at which the current Icl(T) is zero even if a turn on signal IN at a high logic level is present. As a matter of fact, should the turn on signal be always high, the temperature TSTOP would be never attained. This is so because the circuit will eventually reach a temperature lower than TSTOP, at which point the power dissipated in the circuit equals the rate of heat dissipation in the environment.
The temperature TSTOP should be lower than the maximum junction temperature that may be tolerated by the integrated power transistor and/or be lower than the temperature at which unacceptable variations of the bandgap voltage, customarily used by the circuit as reference voltage, would take place. Preferably the temperature TSTOP is lower than 190xc2x0 C. and the temperature TSTART, which is prescribed by specifications, is not lower than 150xc2x0 C.
The difference xcex94T=TSTOPxe2x88x92TSTART determines the value of the coefficient K1 of the soft thermal shut down circuit. It is not possible to set a relatively low TSTOP, i.e. close to TSTART, by setting a certain value K1, because of the problem of global stability of the system. In fact, should K1 be too great there would be abrupt variations of Ib with temperature. This would cause a consequent undesired oscillation of the output voltage Vc in proximity of the thermal equilibrium temperature. For this reason the values of K1 should be limited such to establish a TSTOP preferably of about 180xc2x0 C. to 190xc2x0 C.
The circuit of FIG. 1A may be improved by using an auxiliary thermal sensor TON THERMAL SENSOR, as depicted in FIG. 2A. Diagrams of the main signals as a function of the time and of the temperature are also depicted in FIG. 2B. The auxiliary thermal sensor enables the soft thermal shut down circuit when a certain temperature TON has been overcome by the integrated circuit. In this way, the soft thermal shut down circuit does not interfere in the normal functioning of the device if the circuit temperature is lower than the activation temperature.
These well known approaches do not fully address the problem of the instability of Vc. At best, the oscillations of the collector voltage Vc are limited in order to prevent inducing overvoltages that may produce sparks at the plugs in the secondary circuit of the coil. For example, the detected functioning of the commercially available device VB025 of STMicroelectronics, a functional diagram of which is depicted in FIG. 2A, after a turn on pulse IN (Ch1) lasting relatively for a long time (80 seconds), is illustrated in FIG. 8. Evident oscillations of the collector voltage (Ch3) can be noticed, when the collector current (Ch4, 2A/div), that is the current circulating in the primary circuit of the coil, diminishes.
It has been found and is the object of the present invention to provide a thermal control circuit and a related method of soft thermal shut down of an integrated power transistor that reduces the amplitude of the oscillations of the collector voltage of the power transistor. This is obtained by employing a current amplifier A1(T)I having a variable gain that is controlled by the SOFT THERMAL SHUT DOWN circuit. The invention is directed to reducing the gain of the amplifier when the temperature increases, instead of reducing the current provided to the amplifier through the soft thermal shut down circuit, as in the known devices. In this manner, the value of Ib is reduced, but differently from the circuit of the prior art. At the same time also its oscillations, due for example to the input noise of the amplifier, are reduced.
More precisely, the thermal control circuit is for an integrated power transistor and comprises a current generator controlled by a turn on signal, a sensing resistance in series with the power transistor, a current limiter enabled when the voltage drop at the nodes of the sensing resistance overcomes a certain value, and a current amplifier coupled to the output node of the controlled current generator producing a drive current that is injected on the control node of the power transistor. The thermal control circuit may also include a soft thermal shut down circuit whose state of conduction increases as the temperature increases thereby progressively reducing the drive current of the power transistor.
The circuit of the invention controls the voltage on the power transistor in a significantly more effective manner than the known circuits because the current amplifier has a gain that varies as a function of the state of conduction of the soft thermal shut down circuit, and, therefore, as a function of the temperature.
A further object of the invention is to provide a method of soft thermal shut down of a power transistor that allows a reduction of the oscillations of the collector voltage. This method, implemented with a thermal control circuit of the invention, substantially includes reducing progressively the gain of the drive current amplifier as the temperature increases until a thermal equilibrium is reached.